Conventional cellular communication systems adopt a frequency reuse plan. Generally speaking, system antennas are erected at spaced apart locations. Each system antenna, along with transmitter power, receiver sensitivity, and geographical features, defines a cell. A cell is a geographical area on and near the surface of the earth within which communications may take place via a subscriber unit having predetermined operating characteristics and via the cell's antenna In a cellular system that efficiently uses the spectrum allocated to it, system antennas are located to minimize overlap between their respective cells and to reduce gaps between the cells.
The spectrum allocated to a conventional cellular system is divided into a few discrete portions, typically frequency bands. Each cell is allocated only one of the discrete portions of the spectrum, and each cell is preferably surrounded by cells that use other discrete portions of the spectrum. Communications within a cell use only the discrete portion of the spectrum allocated to the cell, and interference between communications taking place in other nearby cells is minimized because communications in such nearby cells use different portions of the spectrum. Co-channel cells are cells that reuse the same discrete portion of spectrum. To minimize interference, the frequency reuse plan spaces co-channel cells a minimum predetermined distance apart.
A cellular communication system which places antennas in moving orbits around the earth faces particular problems related to distributing discrete portions of the allocated spectrum to various cells. Due to the approximately spherical shape of the earth, cells which do not overlap in one region of the earth, such as the equator, may very well overlap in other regions, such as polar regions. When cells overlap, the co-channel cells that the overlapping cells are spacing apart reside closer together than permitted by the spectrum reuse plan. Interference between communications taking place in such closely spaced co-channel cells becomes more likely.
In addition, when antennas move relative to each other, the overlap between cells changes as a function of time. Any allocation of discrete portions of the spectrum to the cells remains valid only until relative movement of the antennas causes the overlap between the cells to change.